The development of the upper limb begins during the fourth week of in utero life, when a limb bud consisting of undifferentiated mesenchymal cells encased in ectoderm develops. By 9 weeks, the bud has developed into an arm and hand with identifiable digits, and by 12 weeks, the digits have differentiated. Growth and differentiation are under the control of signal regions at the tip of the developing limb, with complicated interactions and feedback systems (Figure 16.1). Induction of mesenchymal cells in the ‘progress zone’ at the tip of the developing limb occurs under the influence of specific zones. The apical ectodermal ridge (AER) is the director of growth in the proximo-distal axis (excision of the AER results in a limb stump only, and transplantation may result in a duplicate limb).